The present invention relates to the art of thread forms, and more particularly to a thread structure for use on percussion drilling parts, such as extension rods, and coupling sleeves for connecting extension rods.
Percussion drilling operations ordinarily employ a number of longitudinal extension drill rods joined at their ends by coupling sleeves. These extension drill rods characteristically have external thread structures which cooperate with the internal thread structures of the coupling sleeves to provide a connection therebetween. These thread forms will be referred to herein as having top crest portions and flank portions, the lower ends of which flanks are interconnected by root portions.
One problem arising with known thread forms used for percussion drilling parts is that the threads thereof are tightened due to the percussive rotary forces being transmitted during the drilling operation. This, of course, results in a corresponding increase in the torque necessary to uncouple the parts. It has been found that this increase in required uncoupling torque is due to wedging between the cooperating thread structures. In a relatively new or unworn thread, the wedging problem is attributable to the binding between the flank portions of one thread structure and the complementary portions of the cooperating thread structure. In a used or worn thread, the wedging problem is due to binding between the flank and crest portions of the thread structure and the complementary structures.
U.S. Pat. No. 3,645,570 to Johansson et al exemplifies prior art attempts to resolve this problem. The thread structure disclosed therein is profiled in such a way that, upon proper engagement of the cooperating thread structures, the crest and root portions of the respective thread structures do not contact one another. More specifically, Johansson discloses a thread design having abutting flanks which are inclined at 25.degree. to 40.degree. and preferably 35.degree. to the drill rod axis. The crest and root portions are substantially straight and parallel to the drill rod axis and are radially spaced from the complementary portions of the cooperating thread structure. So designed, the possibility of wedging between portions of one thread structure with the root portions of the cooperating thread structure is reduced.
Such thread designs do not, however, cope with the effect of wedging after substantial wear of the thread structure has occurred. In percussion drilling, the threads are subject to both rotary and percussive forces. These percussive forces, which are transmitted by the abutting flanks, cause substantial wear thereof. As the abutting flanks are eroded, the thread structure remaining has a portion which is in effect an extension of the root portion and is substantially parallel to the drill rod axis. Since the crest portion of the cooperating thread abuts this worn extension of the root portion of the eroded flank, wedging develops.
Another common problem occurring during the use of percussion drill rods is that the threads tend to break due to fatigue stresses. It has been found that these stresses generally occur where there is a sharp change in diameter of the rod.
The present invention provides a thread structure featuring low uncoupling torque characteristics which are maintained during the life of the thread. Further, a thread structure in accordance with the present invention minimizes fatigue stresses during use.
The present invention more specifically provides percussion drill rod extensions and couplings with threads having crest portions which are slightly inclined to the drill rod axis. The crest portions are angled away from the abutting flank portions toward the root portions whereby the maximum radial dimension occurs immediately adjacent the abutting flank portion. This avoids extensive contact between the crest portions of the thread and the complementary portion of the cooperating thread structure, even after the abutting flank has been significantly eroded. Thus, uncoupling torque is minimized.
In accordance with another aspect of the present invention, a thread structure is provided which reduces fatigue stress concentration by providing continuously curved root portions. Specifically, the curvature of the root portions meets each of the flank portions on opposite sides of the root portion tangentially.